Include license in documentation

**Is your feature request related to a problem? Please describe.**
The license included in the `LICENSE.md` file should automatically be pulled into the documentation.

**Describe the solution you'd like**
The [Sphinx documentation](https://www.sphinx-doc.org/en/master/usage/markdown.html) recommends using the [MyST-Parser](https://myst-parser.readthedocs.io/en/latest/) extension to include markdown files in rST.

**Describe alternatives you've considered**
At a minimum, the license file should be linked from the documentation. I have also tried simply using the directive `.. include:: ../../LICENSE.md`, but this doesn't render the license well and raises warnings when building the documentation.

**Additional context**
MyST-Parser is not currently included in the ESMValTool community environment at the Met Office, it will need to be installed in order for the above method to work.

Start with LICENSE.md and the existing Sphinx documentation setup. Check how MyST-Parser could include the license or whether a documentation link is more appropriate, then build the documentation to verify the license is rendered or linked without warnings and that the required extension is available in the community environment.

Written by the indexing model from the issue text.